U.S. patent number 10,112,613 [Application Number 15/042,465] was granted by the patent office on 2018-10-30 for systems and methods for managing torque.
This patent grant is currently assigned to Ford Global Technologies, LLC. The grantee listed for this patent is Ford Global Technologies, LLC. Invention is credited to Jonathan Andrew Butcher, Dale Scott Crombez, Ryan Hunt, Deborah Sue Lienau, William David Treharne.
United States Patent |
10,112,613 |
Hunt , et al. |
October 30, 2018 |
Systems and methods for managing torque
Abstract
A method of managing torque at a vehicle standstill includes
outputting torque from a powertrain to satisfy a driver torque
demand. The method also includes, in response to a nonzero torque
demand resulting in vehicle standstill, applying a friction brake
to maintain the vehicle standstill and substantially reducing
output torque of the powertrain during friction brake application.
The method further includes satisfying driver torque demand using
the powertrain and releasing the friction brake in response to the
driver torque demand deviating from the nonzero torque demand by
more than a predetermined amount.
Inventors: |
Hunt; Ryan (Ferndale, MI),
Butcher; Jonathan Andrew (Farmington Hills, MI), Crombez;
Dale Scott (Livonia, MI), Treharne; William David
(Ypsilanti, MI), Lienau; Deborah Sue (Plymouth, MI) |
Applicant: |
Name |
City |
State |
Country |
Type |
Ford Global Technologies, LLC |
Dearborn |
MI |
US |
|
|
Assignee: |
Ford Global Technologies, LLC
(Dearborn, MI)
|
Family
ID: |
59410611 |
Appl.
No.: |
15/042,465 |
Filed: |
February 12, 2016 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
|
US 20170232969 A1 |
Aug 17, 2017 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B60W
30/18027 (20130101); B60W 10/184 (20130101); B60W
10/04 (20130101); B60W 10/02 (20130101); B60W
10/08 (20130101); B60T 8/17 (20130101); B60W
20/00 (20130101); B60W 30/18054 (20130101); B60W
30/18118 (20130101); B60W 20/40 (20130101); B60T
2201/06 (20130101); B60W 2540/10 (20130101); B60T
2260/04 (20130101); Y02T 10/72 (20130101); B60W
2510/1005 (20130101); B60W 2720/30 (20130101); B60W
2710/083 (20130101); B60K 6/20 (20130101); B60W
2520/04 (20130101); B60W 2720/10 (20130101); Y02T
10/7258 (20130101); Y02T 10/7275 (20130101) |
Current International
Class: |
B60W
30/18 (20120101); B60W 20/40 (20160101); B60W
10/08 (20060101); B60W 10/04 (20060101); B60W
10/02 (20060101); B60T 8/17 (20060101); B60W
20/00 (20160101); B60W 10/184 (20120101); B60K
6/20 (20071001) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2757005 |
|
Jul 2014 |
|
EP |
|
2013112179 |
|
Aug 2013 |
|
WO |
|
Primary Examiner: Hlavka; David J
Attorney, Agent or Firm: Kelley; David B. Brooks Kushman
P.C.
Claims
What is claimed is:
1. A method comprising: outputting torque from a powertrain to
satisfy a driver torque demand; responsive to a nonzero torque
demand resulting in vehicle standstill, applying a friction brake
to maintain the vehicle standstill and reducing output torque of
the powertrain during friction brake application; maintaining
application of the friction brake for a predetermined duration of
time following a shift from a motive transmission gear to a
non-motive transmission gear; and responsive to the driver torque
demand deviating from the nonzero torque demand by more than a
predetermined amount, satisfying driver torque demand using the
powertrain and releasing the friction brake.
2. The method of claim 1 wherein applying the friction brake occurs
after a vehicle speed is zero for a predetermined duration of
time.
3. The method of claim 1 wherein applying the friction brake
produces a resistive torque that exceeds a standstill torque by a
predetermined amount.
4. A vehicle comprising: a powertrain configured to output torque
to satisfy driver torque demand; and a controller programmed to
responsive to a nonzero torque demand resulting in vehicle
standstill, apply a friction brake to maintain the vehicle
standstill and reduce output torque of the powertrain during
friction brake application; maintain application of the friction
brake for a predetermined duration of time following a shift from a
motive transmission gear to a non-motive transmission gear; and
responsive to the driver torque demand deviating from the nonzero
torque demand by more than a predetermined amount, satisfying
driver torque demand using the powertrain and releasing the
friction brake.
5. The vehicle of claim 4 wherein the applying occurs after a
vehicle speed is zero for a predetermined duration of time.
6. The vehicle of claim 4 wherein the applying produces a resistive
torque that exceeds a standstill torque by a predetermined amount.
Description
TECHNICAL FIELD
The present disclosure relates to managing torque output of an
electric powertrain.
BACKGROUND
In a conventional internal combustion powertrain vehicle it may be
desirable to output positive wheel torque to hold the vehicle at a
standstill condition. This condition may occur, for example, when a
driver attempts to hold the vehicle position at a standstill on an
incline using only the accelerator pedal. The powertrain may
include a torque converter which allows the engine output shaft to
spin while the drive shaft remains still, until sufficient torque
is transferred by the fluid connection of the torque converter to
move the vehicle forward. Also, the fluid connection of a torque
converter allows the engine to spin at vehicle standstill without
causing substantial wear on powertrain components.
A disconnect clutch can also be used to vary the torque transferred
from the engine to the drive shaft. For example varying the clutch
pressure in order to "slip" the clutch can vary the amount of
torque transferred to the drive shaft. However slipping the clutch
can increase clutch surface temperature and in turn affect clutch
durability.
SUMMARY
A method includes outputting torque from a powertrain to satisfy a
driver torque demand. The method also includes, in response to a
nonzero torque demand resulting in vehicle standstill, applying a
friction brake to maintain the vehicle standstill and substantially
reducing output torque of the powertrain during friction brake
application. The method further includes satisfying driver torque
demand using the powertrain and releasing the friction brake in
response to the driver torque demand deviating from the nonzero
torque demand by more than a predetermined amount.
A vehicle includes a powertrain to output torque at a wheel and a
friction brake to resist wheel rotation. The vehicle also includes
a controller programmed to apply the friction brake and deactivate
the powertrain in response to receiving an accelerator pedal input
causing the powertrain to output a nonzero standstill torque
resulting in zero vehicle speed while in a motive transmission
gear.
A vehicle includes a powertrain to provide torque at a wheel and a
friction brake to resist rotation of the wheel. The vehicle also
includes a controller programmed to apply the friction brake to
hold vehicle position and substantially reduce output torque of the
powertrain in response to receiving a driver torque demand less
than a first torque threshold.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a hybrid vehicle.
FIG. 2 is a flowchart of a method of managing torque at a vehicle
standstill condition.
FIG. 3 is a plot of torque output for a drive train in a standstill
mode.
DETAILED DESCRIPTION
Embodiments of the present disclosure are described herein. It is
to be understood, however, that the disclosed embodiments are
merely examples and other embodiments can take various and
alternative forms. The figures are not necessarily to scale; some
features could be exaggerated or minimized to show details of
particular components. Therefore, specific structural and
functional details disclosed herein are not to be interpreted as
limiting, but merely as a representative basis for teaching one
skilled in the art to variously employ the present invention. As
those of ordinary skill in the art will understand, various
features illustrated and described with reference to any one of the
figures can be combined with features illustrated in one or more
other figures to produce embodiments that are not explicitly
illustrated or described. The combinations of features illustrated
provide representative embodiments for typical applications.
Various combinations and modifications of the features consistent
with the teachings of this disclosure, however, could be desired
for particular applications or implementations.
FIG. 1 depicts a plug-in hybrid-electric vehicle (PHEV). A PHEV 112
may comprise one or more electric machines 114 mechanically coupled
to a hybrid transmission 116. The electric machines 114 may be
capable of operating as a motor or a generator. In addition, the
hybrid transmission 116 is mechanically coupled to an engine 118.
The hybrid transmission 116 is also mechanically coupled to a drive
shaft 120 that is mechanically coupled to the wheels 122. The
electric machines 114 can provide propulsion and deceleration
capability either while the engine 118 is operated or turned off.
The electric machines 114 are capable of operating as generators
and provide fuel economy benefits by recovering energy that would
normally be lost as heat in a friction braking system. The electric
machines 114 may additionally impart a reaction torque against the
engine output torque to generate electricity for recharging a
traction battery the while the vehicle is operating. The electric
machines 114 may further reduce vehicle emissions by allowing the
engine 118 to operate near the most efficient speed and torque
ranges. When the engine 118 is off, the PHEV 112 may be operated in
electric-only mode using the electric machines 114 as the sole
source of propulsion. The output control precision achievable by
the electric machines 114 eliminates the need for a torque
converter that is required to bypass torque in a vehicle standstill
condition in a conventional internal combustion powertrain. As
discussed in greater detail below, in a standstill condition of a
hybrid vehicle according to the present disclosure, the engine 118
may be disabled or disconnected and no power supplied to the
electric machines 114.
A traction battery, or battery pack, 124 stores energy that can be
used by the electric machines 114. The battery pack 124 typically
provides a high-voltage direct current (DC) output. One or more
contactors 142 may isolate the traction battery 124 from a DC
high-voltage bus 154A when opened and couple the traction battery
124 to the DC high-voltage bus 154A when closed. The traction
battery 124 is electrically coupled to one or more power
electronics modules 126 via the DC high-voltage bus 154A. The power
electronics module 126 is also electrically coupled to the electric
machines 114 and provides the ability to bi-directionally transfer
energy between AC high-voltage bus 154B and the electric machines
114. For example, a traction battery 124 may provide a DC current
while the electric machines 114 may operate using a three-phase
alternating current (AC) to function. The power electronics module
126 may convert the DC current to a three-phase AC current to
operate the electric machines 114. In a regenerative mode, the
power electronics module 126 may convert the three-phase AC current
output from the electric machines 114 acting as generators to the
DC current compatible with the traction battery 124. The
description herein is equally applicable to a pure electric
vehicle.
In addition to providing energy for propulsion, the traction
battery 124 may provide energy for other vehicle electrical
systems. A vehicle 112 may include a DC/DC converter module 128
that is electrically coupled to the high-voltage bus 154. The DC/DC
converter module 128 may be electrically coupled to a low-voltage
bus 156. The DC/DC converter module 128 may convert the
high-voltage DC output of the traction battery 124 to a low-voltage
DC supply that is compatible with low-voltage vehicle loads 152.
The low-voltage bus 156 may be electrically coupled to an auxiliary
battery 130 (e.g., 12V battery). The low-voltage systems 152 may be
electrically coupled to the low-voltage bus 156. The low-voltage
system 152 may include various controllers within the vehicle
112.
The traction battery 124 of vehicle 112 may be recharged by an
external power source 136. The external power source 136 may be a
connection to an electrical outlet. The external power source 136
may be electrically coupled to a charger or electric vehicle supply
equipment (EVSE) 138. The external power source 136 may be an
electrical power distribution network or grid as provided by an
electric utility company. The EVSE 138 may provide circuitry and
controls to regulate and manage the transfer of energy between the
power source 136 and the vehicle 112. The external power source 136
may provide DC or AC electric power to the EVSE 138. The EVSE 138
may have a charge connector 140 for plugging into a charge port 134
of the vehicle 112. The charge port 134 may be any type of port
configured to transfer power from the EVSE 138 to the vehicle 112.
The charge port 134 may be electrically coupled to a charger or
on-board power conversion module 132. The power conversion module
132 may condition the power supplied from the EVSE 138 to provide
the proper voltage and current levels to the traction battery 124.
The power conversion module 132 may interface with the EVSE 138 to
coordinate the delivery of power to the vehicle 112. The EVSE
connector 140 may have pins that mate with corresponding recesses
of the charge port 134. Alternatively, various components described
as being electrically coupled or connected may transfer power using
a wireless inductive coupling.
Other vehicle configurations may benefit from aspects of the
present disclosure. In addition to illustrating a plug-in hybrid
vehicle, FIG. 1 can be representative of a battery electric vehicle
(BEV) configuration if the engine 118 is removed. FIG. 1 could also
represent a traditional hybrid electric vehicle (HEV) or a
power-split hybrid electric vehicle if the components 132, 134,
136, 138, and 140 that are related to plug-in charging are removed.
A further example vehicle configuration which may benefit from
aspects disclosed herein is a conventional vehicle having the
internal combustion engine 118 as the sole propulsion source. Such
a conventional vehicle may not include an electric machine 114 or
traction battery 124 to provide vehicle traction but may contain a
slipping clutch that could overheat or wear, similar to the
electric machine overheat condition.
One or more friction brakes 144 may be provided for resisting
rotation of the wheels in order to decelerate the vehicle 112 or
prevent vehicle movement altogether. The friction brakes 144 may be
hydraulically actuated, electrically actuated, or some combination
thereof. The friction brakes 144 may be a part of a brake system
150 which includes other components to operate the friction brakes
144. For simplicity, the figure depicts a single connection between
the brake system 150 and one of the friction brakes 144. A
connection between the brake system 150 and the other friction
brakes 144 is implied. The brake system 150 may also include a
controller to monitor and coordinate the brake system 150. The
brake system 150 may monitor the brake components and regulate
pressure applied by the friction brakes 144 to control vehicle
deceleration. The brake system 150 may respond to driver commands
via a brake pedal. As discussed in more detail below, the brake
system may also operate autonomously to implement advanced features
such as stability control and vehicle hold on an incline. The
controller of the brake system 150 may implement a method of
applying a predetermined brake force when requested by another
controller or sub-function.
One or more high-voltage electrical loads 146 may be coupled to the
high-voltage bus 154. The high-voltage electrical loads 146 may
have an associated controller that operates and controls the
high-voltage electrical loads 146 when appropriate. For example,
high-voltage loads 146 may include compressors and electric heaters
that are part of a climate control system.
The various components discussed may have one or more associated
controllers to control and monitor the operation of the components.
The controllers may communicate via a serial bus (e.g., Controller
Area Network (CAN)) or via discrete conductors. In addition, a
system controller 148 may be present to coordinate the operation of
the various components.
During an ignition-off condition, the contactors 142 may be in an
open state so that the traction battery 124 does not provide power
to the high-voltage bus 154. During the ignition-off condition, the
traction battery 124 may be decoupled from the auxiliary battery
130. During the ignition-off condition, selected electronic modules
(e.g., low-voltage loads 152) may remain active. For example, a
theft-deterrent system and a remote keyless entry system may
continue to be active. The active systems may draw current from the
auxiliary battery 130. In some configurations, low-voltage loads
152, such as lamps, may be accidently left in an active condition
and draw current from the auxiliary battery 130, which may increase
a rate of discharge of the auxiliary battery 130. During the
ignition-off condition, the low-voltage loads 152 may be configured
to minimize current draw.
System controller 148, although represented as a single controller,
may be implemented as one or more controllers. The controller 148
may monitor operating conditions of the traction battery 124, the
power conversion module 132, the electric machine 114, and the
brake system 150. The traction battery 124 includes a current
sensor to sense a current that flows through the traction battery
124. The traction battery 124 also includes a voltage sensor to
sense a voltage across terminals of the traction battery 124. The
voltage sensor may output a signal indicative of the voltage across
the terminals of the traction battery 124. The traction battery
current sensor may output a signal of a magnitude and direction of
current flowing into or out of the traction battery 124.
The power conversion module 132 also includes a current sensor to
sense a current that flows from the EVSE 138 to the traction
battery 124. The engine 118 coupled to the electric machine 114
generates an AC current that is converted to a DC current by the
power electronics module 126. The engine 118 may be controlled by a
powertrain control module having at least one controller in
connection with the system controller 148. The current sensor of
the power conversion module 132 may output a signal indicative of a
magnitude and direction of current flowing from the EVSE 138 to the
traction battery 124.
The controller 148 may include a processor that controls at least
some portion of the operation of the controller 148. The processor
allows onboard processing of commands and routines. The processor
may be coupled to non-persistent storage and persistent storage. In
an illustrative configuration, the non-persistent storage is random
access memory (RAM) and the persistent storage is flash memory. In
general, persistent (non-transitory) storage can include all forms
of storage that maintain data when a computer or other device is
powered down.
The current sensor and voltage sensor outputs of the traction
battery 124 are provided to the controller 148. The controller 148
may be programmed to compute a state of charge (SOC) based on the
signals from the current sensor and the voltage sensor of the
traction battery 124. Various techniques may be utilized to compute
the state of charge. For example, an ampere-hour integration may be
implemented in which the current through the traction battery 124
is integrated over time. The state of charge may also be estimated
based on the output of the traction battery voltage sensor 104. The
specific technique utilized may depend upon the chemical
composition and characteristics of the particular battery.
The controller 148 may also be configured to monitor the
temperature of the traction battery 124 so as to avoid or mitigate
undesirable operating temperatures.
Under certain circumstances, a vehicle can in a situation where
positive wheel torque is demanded without a desire for the vehicle
to move. This may occur, for example, when the driver attempts to
hold the vehicle position at a standstill while on an incline by
using only the accelerator pedal. This condition may also occur or
when one wheel is in a hole and is attempting to get out, or when
the driver attempts to slowly climb a curb. In a conventional
automatic transmission, the torque converter bypasses torque output
of the engine allowing conditions where the engine spins while the
drivetrain remains still. Once enough torque builds up at the
torque converter fluid connection, the vehicle moves forward.
However, in a hybrid power split or all-electric powertrain as
described in the present disclosure, the absence of a torque
converter causes the three-phase electric motor to apply torque in
response to demand indicated by the accelerator pedal, even though
the rotor is not moving. In a permanent magnet electric machine,
this could cause current to be more heavily concentrated in one
phase of both the inverter and the motor. Concentrated current in
one phase can cause the electric machine (or the inverter) to
rapidly overheat. In turn, the overheating can cause the vehicle to
be incapable of launching from the standstill, or even shutdown the
powertrain based on the designed hardware protection strategies. In
the case of a slipping clutch it may be similarly undesirable to
incur extended periods of rotating the engine shaft when the
vehicle is at a standstill. This condition increases heat generated
at the clutch and accelerates component wear.
The controller 148 is programmed to detect when certain standstill
conditions are met, and then issue commands to cause the friction
brake to automatically apply resistive torque to hold the vehicle
at a standstill instead of using powertrain torque to accomplish
this result. Relieving the electrical load from the electric
machine can prevent overheating of the powertrain due to torque
output while the rotor speed is held at substantially zero. The
output torque of the powertrain may be substantially reduced to
avoid overheating of components related to holding vehicle position
in a standstill condition.
According to aspects of the present disclosure the control system
will enter a friction brake standstill mode if the vehicle is in a
motive gear and the output torque at the one or more wheels is
positive while the vehicle speed is substantially zero. The
controller will then command the brake system to engage the
friction brakes based on the current grade. Once resistive brake
torque has been applied, the system will then reduce the powertrain
torque to zero. The controller may also set a powertrain output
torque value associated with the amount of torque required to hold
the vehicle at a standstill. Determining the standstill torque each
time the standstill mode is entered accounts for any of a range of
different incline angles as well as other vehicle operating
conditions such as a curb climb or moving the vehicle wheels to
exit a significant depression or hole. Once standstill mode is
entered, the controller continues to monitor the driver torque
request until it determines a driver's intention to exit the
standstill mode. In alternative embodiments, a grade sensor may be
provided to output a signal to inform the controller when the
vehicle is on an incline grade.
FIG. 2 is a flowchart of a method 200 to automatically supplant
powertrain output torque with friction brake resistive torque
during certain vehicle standstill conditions. At step 202, the
controller assesses the transmission mode to determine whether or
not the vehicle is in a motive gear. For example, each of "drive,"
"reverse," "low," and "sport" transmission modes may be considered
motive modes and allow for the controller to engage friction brake
standstill mode.
If the vehicle transmission is in a motive mode at step 202, the
controller considers the driver torque demand at step 204. Driver
torque demand may be determined by the amount of input at the
accelerator pedal. For example the accelerator pedal angle may
indicate a driver's intent to hold the vehicle at standstill
without accelerating. Additionally, even at zero vehicle speed
there may be a positive torque demand value, corresponding to a
simulated "creep" torque such as that found in a conventional
transmission having a torque converter.
If there is driver accelerator pedal input indicative of a torque
demand, the controller activates the electric machine. At step 206,
the controller may issue a command to provide current to the
electric machine causing it to provide output torque at the output
shaft corresponding to the driver torque demand.
The controller monitors vehicle speed and determines whether the
driver is applying just enough accelerator pedal input to indicate
a desire to hold the vehicle at a standstill. If at step 208 the
vehicle speed is substantially zero, the vehicle is determined to
be at standstill. The controller then assesses the amount of time
held in this condition. At step 210 the controller starts a timer
by setting a time t0 as a baseline to begin counting. At step 212
the counter increments the time step to track the duration for
which the vehicle is held at standstill using torque output from
the electric machine.
If at step 214 the duration of time over which the vehicle is held
at standstill exceeds a time threshold t.sub.Standstill, the
controller enters the friction brake standstill portion of the
control algorithm. If at step 214 the time has not exceeded the
time threshold t.sub.Standstill, the controller continues allow
output torque from the electric machine and monitors the cumulative
time over which the vehicle is held at standstill.
At step 216 the controller stores a value of standstill torque
T.sub.standstill that is based on the driver torque demand at the
accelerator pedal which corresponds to vehicle speed being
substantially zero over a predetermined duration of time. In
alternative embodiments, the controller may rely on a signal from a
sensor indicating an incline angle of the vehicle. In response to a
vehicle standstill on an incline grade greater than a predetermined
threshold, the controller may store the powertrain output value
corresponding to zero vehicle speed on the incline grade.
At step 218 the controller issues a command to cause application of
at least one friction brake to resist wheel rotation of one or more
vehicle wheels. As discussed in more detail below, the amount of
brake pressure may be based on generating brake resistive torque
that exceeds the standstill torque by a predetermined amount. More
specifically, the friction brakes may be applied with sufficient
pressure to correspond to a target brake resistive torque
T.sub.brake, which is selected each time entering standstill mode
based on the determination of T.sub.standstill. At step 220 the
controller issues a command to reduce current supplied to the
electric machine. Output torque of the electric machine is reduced
to zero. The controller may exit friction brake standstill mode
either by a significant change in driver accelerator pedal input or
if the vehicle is shifted out of the original motive transmission
gear.
At step 222 if the powertrain is shifted out of the original motive
gear the controller prepares to exit friction brake standstill
mode. In one example, shifting from either "drive" or "reverse"
into "park" may trigger such a transition out of standstill mode.
In another example, a shift from "drive" to "reverse" may indicate
a driver intention to change direction and similarly prompt an exit
from standstill mode. Although a user input to change transmission
gears is received, the controller causes a predetermined delay
t.sub.Shift at step 226 prior to re-assessing criteria for
remaining in standstill mode in order to allow for completion of
the gear shift before releasing the friction brake. The
predetermined delay time may be implemented when shifting gears
before applying the gear or new torque map as well as when exiting
the standstill mode. Conversely, in further examples, a shift
between more closely related drive modes may not cause a delay
prior to exiting standstill mode. That is, the controller may
continuously monitor torque demand with regard to the transmission
gear change. In one example, a shift from "drive" into a related
"low" or "sport" gear may not automatically prompt a predicate
delay prior to exiting standstill mode.
If at step 222 the transmission remains in the same motive gear as
when it entered into standstill mode the controller monitors
whether a change in accelerator position indicates an intention
driver by the driver to move the vehicle. The controller
establishes a predetermined torque band with limits above and below
the powertrain torque demand corresponding to T.sub.standstill used
to hold the vehicle at standstill. While the powertrain torque
demand remains in this region, the vehicle will remain in
standstill mode, and the brakes will remain applied. Use of a
torque band about the powertrain torque demand corresponding to
T.sub.standstill is designed so that the vehicle remains at a
standstill while the driver holds the accelerator pedal in
relatively the same position, and accounts for small adjustments or
twitches in the driver's foot.
If at step 230 the driver pedal input corresponds to a torque
demand which is greater than a torque threshold equal to
T.sub.standstill plus a predetermined buffer amount, the controller
issues a command at step 234 to reactivate the electric machine. If
at step 230 the driver pedal input corresponds to a torque demand
that is less than the upper end of the torque band, the controller
assesses the lower end of the torque band at step 232. That is, the
controller determines at step 232 whether driver pedal input
corresponds to a torque request that is less than a torque
threshold equal to T.sub.standstill minus a predetermined buffer
amount. If the torque request decreases to less than the lower
torque band threshold, the controller issues a command at step 234
to reactivate the electric machine to satisfy the torque request.
The lower end of the torque band may relate to a low torque request
where the driver intends for the powertrain to output less than
T.sub.standstill, for example to purposely allow the vehicle to
roll down the incline.
Once powertrain torque is increased to equal to driver demanded
torque, the controller will disengage the standstill mode. At step
228 the controller issues a command to release the friction brake.
The vehicle then resumes normal powertrain operation outside of
friction brake standstill mode.
The controller repeats the loop of method 200 to account for
traffic conditions including high frequency stop and go occurrences
when the vehicle is on an incline. In such cases driver may wish to
hold the vehicle a standstill repeatedly while waiting for the next
opportunity to move forward. Therefore the controller is programmed
to continually monitor duration of standstill each time the vehicle
is held at standstill using torque output from the electric
machine. When practical (i.e., after the predetermined time
duration discussed above) the controller employs the friction brake
to relieve the electric machine from outputting torque to maintain
a vehicle standstill condition.
FIG. 3 include plot 300 which provides a graphical representation
of aspects of the present disclosure. The plot represents the
application of powertrain output torque and friction brake torque
once the vehicle has entered friction brake standstill mode.
Horizontal axis 302 represents demanded torque which may be based
on an accelerator pedal position as influenced by a driver.
Vertical axis 304 represents applied torque, whether output torque
provided by the powertrain or resistive torque applied by the
friction brake.
As discussed above, the controller determines a driver intention to
hold the vehicle at standstill indicated by a driver torque demand
corresponding to a torque output that causes substantially zero
vehicle speed. The then controller enters friction brake standstill
mode. Curve 306 represents resistive torque applied by the vehicle
brake system. Curve 308 represents powertrain output torque. In the
case of a vehicle on an incline, the resistive torque from the
brake system preventing backward roll is in the same direction as
the propulsion torque in the uphill direction which also opposes
backward roll.
Once the controller determines standstill torque 310, the friction
brake applies all torque to hold the vehicle position and the
powertrain is deactivated to provide zero torque. The controller
also establishes a buffer range, or torque band, 312
bi-directionally straddled about the standstill torque such that as
long as driver demand remains in the range the torque resisting
movement is provided by the friction brake. Once the driver torque
demand shifts corresponding to torque demand that is outside of the
bidirectional torque band 312, the electric machine is reactivated.
On the upper end of the torque band, increased torque demand may
indicate a driver's intention to propel the vehicle forward. On the
lower end of the torque band, reduced torque demand may indicate a
driver's intention to allow the vehicle to roll backward down the
incline. In either case the friction brake is released and normal
operation of the powertrain is resumed.
While the bidirectional torque band 312 is depicted as
symmetrically balanced about the standstill torque 310, there may
be conditions under which it is desirable to achieve a more rapid
response on either the upper end or the lower end of the torque
band compared to the other. In one example (not shown), the upper
end of the band may be closer to the standstill torque to provide a
more rapid acceleration response and exit friction brake standstill
mode as the driver depresses the accelerator pedal.
Also discussed above, the resistive brake torque applied during
standstill mode is set to a value greater than the torque value of
the upper end of the torque band. In alternative embodiments, the
resistive torque applied by the friction brake is tailored to match
torque demand. In this way transitions into and out of standstill
mode may be smoother when the friction brake is released in favor
of powertrain output torque. In additional embodiments, the
resistive brake torque is set to a value determined by a table
using a measurement of the incline grade of the vehicle, plus a
predetermined additional torque value.
The processes, methods, or algorithms disclosed herein can be
deliverable to/implemented by a processing device, controller, or
computer, which can include any existing programmable electronic
control unit or dedicated electronic control unit. Similarly, the
processes, methods, or algorithms can be stored as data and
instructions executable by a controller or computer in many forms
including, but not limited to, information permanently stored on
non-writable storage media such as ROM devices and information
alterably stored on writeable storage media such as floppy disks,
magnetic tapes, CDs, RAM devices, and other magnetic and optical
media. The processes, methods, or algorithms can also be
implemented in a software executable object. Alternatively, the
processes, methods, or algorithms can be embodied in whole or in
part using suitable hardware components, such as Application
Specific Integrated Circuits (ASICs), Field-Programmable Gate
Arrays (FPGAs), state machines, controllers or other hardware
components or devices, or a combination of hardware, software and
firmware components.
While exemplary embodiments are described above, it is not intended
that these embodiments describe all possible forms encompassed by
the claims. The words used in the specification are words of
description rather than limitation, and it is understood that
various changes can be made without departing from the spirit and
scope of the disclosure. As previously described, the features of
various embodiments can be combined to form further embodiments of
the invention that may not be explicitly described or illustrated.
While various embodiments could have been described as providing
advantages or being preferred over other embodiments or prior art
implementations with respect to one or more desired
characteristics, those of ordinary skill in the art recognize that
one or more features or characteristics can be compromised to
achieve desired overall system attributes, which depend on the
specific application and implementation. These attributes can
include, but are not limited to cost, strength, durability, life
cycle cost, marketability, appearance, packaging, size,
serviceability, weight, manufacturability, ease of assembly, etc.
As such, embodiments described as less desirable than other
embodiments or prior art implementations with respect to one or
more characteristics are not outside the scope of the disclosure
and can be desirable for particular applications.
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